Austenitic ferrous alloys and articles made thereof



Patented Mar. 16, 1937 UNITED STATES AUSTENITIC FERROUS ALLOYS AND ARTLCLES MADE THEREOF HaroldD. Newell, Beaver Falls, Pa., assignor to TheBabcock & Wilcox Tube Company, West Mayfield, Pa., a corporation ofPennsylvania No Drawing. Application May 29 1930, Serial No. 457,517

3 Claims.

This invention relates to austenitic ferrous alloys and articles madetherefrom, and more particularly corrosion resistant austeniticchromium-iron alloys for use at elevated tempera- 5 tures, the object ofthe invention being to provide articles of such alloys which shall to ahigh degree resistcorrosion and maintain their physical properties whenused at elevated temperatures and exposed to the action of corrodingsubstances. Austenitic chromium-iron alloys, and especiallychromiumnickel-iron alloys of the class containing 10 to 22 percentchromium and 7 to percent nickel, with a low carbon content, usually notoverabout .15 percent and more generally 15 of about .07 percentmaximum, and sometimes containing also small quantities of additionalelements such as molybdenum, silicon, vanadium, tungsten, and titanium,have come into quite extensive use for various purposes where resistanceto corrosion combined with a comparatively great strength and a gooddegree of ductility and toughness is desirable. The invention has beenmade more especially with the idea of improving alloysof this generalclass, but applies toother austenitic ferrous alloys in which chromiumis used to give corrosion resistance. It has generally been consideredthat to develop the greatest corrosion resistance combined witharelatively high degree of ductility and toughness in these alloys, theyshould be treated by beating them to a temperature suificient to obtaincomplete solubility of the carbon and then cooling at a ratesufficiently to retain the carbon in solid solution; and. from iiOOto1260 degrees C. has been generally accepted as the best temperaturerange for such treatment, the

articles being preferably quenched in Water. There seems to be noquestion but that the corrosion resistance, ductility, and toughness ofthese alloys are best after such treatment and will remain so at lowtemperatures.

It has been found, however, that when these alloys so treated have beenused at elevated temperatures above 900 degrees F. the ductility of themetal becomes impairedto a substantial de 'gree, even when the metal isnot exposed to corrosive substances, and that the corrosion resistanceof the metal is very greatly impaired, so much so that when exposed tocorrosive attack the metal is liable to such deterioration in itsphysical structure as to be entirely unsuitable and. dangerous to useunder conditions of any considerable strain or sudden stress. Tubes ofthis material heat treated by quenchim rom the 1100 to 1200 degree C.range have been known to burst, after service at a temperature somewhatabove 900 degrees F., under pressure well below what the tube shouldhave withstood, and such bursting has taken place without warning, thatis, without the bulging which in the case of ordinary carbon steel tubesunder similar conditions serves as a warning that the tube is weakeningand replacement necessary. And in place of a slight split in the tubesuch as finally results in a tube which is properly ductile, thebursting of the tubes referred to has been violent and destructive.

I have discovered that if these alloys are worked to produce a smallgrain size, and are then cooled, either quickly or slowly, from atemperature that isnot much above and may even be within the upperlimits of the range within which carbon precipitation occurs, the metal,while not quite so highly corrosion resistant and possibly not quite soductile, at room temperature, aswhen quenched from a temperature between1100 and 1200 degrees C., will still have a relatively high degree ofcorrosion resistance and will be only slightly less ductile atroomtemperature than when cooled from the higher temperature, and, invaddition, willlose comparatively little of its corrosion resistance andductility and strength as the result of exposure to elevatedtemperatures, even when long continued, such as have been found togreatly impair the ductility and practically destroy the corrosionresistance oi? these alloys when they have been quenched from thepreviously mentioned high temperatures. It apparently does not becomesubject to inter granular corrosion.

If articles made from these alloys are fabricatedby hot rolling or otherhot working, then the desired condition of the metal may be ob tained bygoverning the finishing temperature so as to produce the desired finegrain in the metal. The finishing temperature any working of the metalwhich involves substantial deformation should be above the temperatureat which ductility is substantially reduced, while low enough to obtainthe desired small grain size. It i is usually desirable in the case. ofplain chro mium-nickel4ron-carbonalloys. of usual car-.-

bon content to work as much as possible down ,.wi l l; be secured, bycooling either quickly or slowly immatemperature only slightly above thetemperature of recrystallization, that is, from a temperature of about1800 degrees F., and preferably not substantially over 1850 degrees'F.

The deterioration of these austenitic chromium-nickel-ferrous alloyswhich have been quenched from the generally accepted high heat treatmenttemperatures when used at elevated temperatures is, I believe, due tocarbide precipitation in the grain boundaries. Carbide precipitationoccurs in these alloys regardless of previous heat treatment when theyare heated for suflicient time to temperatures between about 900 degreesF. as a minimum and about 1800 degrees F. as a maximum. At about 1800degrees F., carbon (carbide particles) again starts to go into solution.This temperature range varies somewhat with variations in analysis ofthe alloy. With higher carbon or with substantial silicon, for example,the range extends somewhat above 1800 degrees F. and may extend somewhatbelow 900 degrees F.

When the grain size is small and the carbon content low, carbideprecipitation does not-seriously impair either the corrosion resistance.or the ductility of the alloy; but if the grain size is large, then,evenwith relatively low carbon content, carbide precipitation has a verymarked effect on the metal, resulting in lowering its ductility andlarge loss in corrosion resistance; and the higher the carbon content,the greater are these losses. The heat treating temperature r usedbefore the metal is placed in service determines the grain size and thuspermanently establishes the physical properties and limitations of themetal when used at elevated temperatures. The -range of temperature forwhich alloys of the class referred to are usually considered suitablefor use within the carbon precipitationrange is from about 900 degreesF. to about 1350 degrees F., and it is use within this range whichcauses the greatest loss of corrosion resistance, either during or aftersuch use. Above about 1350 degrees-E, and up to the upper limit of thecarbon precipitation range, they are not generally used under conditionsrequiring maintenance of strength and ductility. Re-heating within thisservice range of 900 degrees to 1350 degrees F., or even considerablyabove 1350 degrees F., does not affect the grain size, and, therefore,does not change the relationship of the initial heat treatmenttemperature as to effect on'physical properties.

Carefully conducted tests have shown that when an alloy of the classreferred to which has been quenched from a temperature sufliciently'high to completely absorb the carbon (for example, the generallyaccepted high heat treatment temperature of from 1100 to 1200 degreesC.) and which is thereby given a large grain structure, is exposed tocorrosive attack after having been subjected in service to a tempera-*ture within a range corresponding approximately degrees R, anintergranular corrosion takes place which is exceedingly detrimental,penetrating more or less deeply into the metal according to relativelyslight and confined to the surface of the metal, and the characteristicsof the metal remaining substantially unaffected. Similar results havebeen observed in actual use of these alloys.

The extensive tests which I have made on different austeniticchromium-nickel-iron alloys of the class referred to, and myobservations of these alloys in use under various conditions, clearlyestablish that the value of the alloys for use at temperatures withinthe carbide precipitation range, and especially when exposed tocorrosive attack, diminishes with increase in grain size and withincrease in carbon content, and that to avoid large grain size in thealloys, heat treatment at the high temperatures most suitable when thealloys are intended for low temperature service should not be used. Thegrain size increases greatly when the alloys are heated above 1900degrees F.

Tests which I have made in connection with this invention were mademostly on alloys containing approximately .06 percent carbon, 18 percentchromium, and 9 percent nickel, with small percentages in theneighborhood of .35 percent of manganese and silicon, but other testsshow clearly that similar results follow with other alloys of the classreferred to and with chromiumnickel-iron alloys containing higherpercentages of the chromium and nickel and with other corrosionresistant chromium-iron alloys.

The explanation of the greatly improved corrosion resistance andretention of physical properties of the fine grained alloys when used attemperatures within the carbon precipitation range, and especially fromabout 900 to about 1350 degrees F., I believe to be as follows. With agiven carbon content in the alloy, a certain quantity of carbideparticles will precipitate when the temperatures prevailing in serviceare sufliciently high; The carbide particles are mostly chromiumcarbide, and the precipitation of these chromium carbide particlesremoves such an amount of chromium from the solid solution near thegrain boundaries as to render these adjacent areas noncorrosionresistant. In coarse grained metal, the

ratio of grain boundary extent to grains is relaonly to surfacecorrosion, but to intergranular to said service range of about 900degrees to 1350 conditions and causing embrittlement of the metal anddestroying its characteristics and usefulness. Small grain material, onthe other hand, while it may show some corrosion loss under the sameconditions after use at such elevated temperatures, does not losecoherence as 75 does the large grained alloy, the corrosion beingcorrosion which may proceed to an extent sufficient to result incomplete loss of usefulness of the metal. If the grain size is small,the ratio of boundaries to-gralns is relatively high, and with theamount of carbon in the alloy sufliciently low, the carbideprecipitation resulting from the service temperature does not result inthe grains being completely surrounded by the carbide particles, but thecarbide particles are mostly'separated and spaced widely apart so thatthe corrosion resistance is lowered locally only and generalintergranular corrosion does not take place. With the grain size small,corrosion resistance, as stated, is somewhat lowered by the carbideprecipitation, but corrosion is relatively slight and is confinedsubstantially to the surface of the metal,

and the metal does not suffer from intergranular corrosion, nor is theductility of the metal seriously afiected. When the grains are large,however, with a similar carbon content, the ductility is substantiallyimpaired even without any corrosion having taken place.

, As the carbon content of the alloy increases, the desired grain sizediminishes, that is, the ratio of boundary extent to amount of carbideparticles must be maintained; otherwise, evenvery fine grained alloysmay not retain their properties under elevated temperature serviceconditions and exposure to corrosion. The carbon. content should,therefore, not be so high that with the small grain size which may beobtained in practice by treating the alloy as hereinbefore pointed out,the grain boundaries willbecome so filled with carbide particles as todestroy the corrosion resistance of the alloy when in service atelevated temperatures. In general, to retain corrosion resistance thecarbon content should not exceed about .15 percent. In some cases,however, it may be desirable to exceed this amount somewhat. When thealloys contain substantial silicon, for example, the silicon tends toform delta iron, which entails a considerable loss in corrosionresistance, and a carbon content slightly higher than .15 percent mightin some cases be desirable in order to retain the austenite phase.

A further advantage of the invention is that the treated alloys do notsuffer appreciable loss in impact value on exposure to temperatureswithin the carbon precipitation range.- It has been found that whileaustenitic iron chromium alloys quenched from a high temperature (1100to 1200 C.) have a better impact value prior to subsequent heating thanthe same alloys treated according to the invention, they show a largeloss of impact value on reheating, whereas the same alloys treatedaccording to the invention maintain nearly their original impact valueon being heated at elevated temperature. especially noticeable with thealloys of relatively high chromium and nickel content.

It is well known that certain elements such as vanadium, molybdenum andtitanium when added to austenitic iron-base alloys in suflicient amountstend to prevent grain growth. In practicing my new method,therefore,suitable additions of such elements to the alloy may sometimes be ofadvantage in aiding in securing and maintaining the desired small grainsize in the alloy.

The term elevated temperature as used in the claims is to be understoodas meaning a temperature within the range of about 900 degrees to about1350 degrees F. The term low carbon as used in the claims means a carboncontent of not over about .15 per cent. The meaning of the expressionssmall grain and fine grain as used in the claims will be clear from theforegoing description, but it may be pointed out that the terms are usedas applied to austenitic alloys the grains of which under allcorresponding conditions are larger than the grains of plain carbonsteel and much larger This is,

than those of tool steel. As examples only, an

alloy having grains of the order of .001 of an inch in diameter wouldbea small grain, or fine grain, alloy as the terms are used herein, butone having grains averaging .01 inch indiameter would not be.

It is recognized that on long continued exposure to temperatures withinthe carbon precipitation range austenitic alloys treated according tothe invention may become partly magnetic.

What is claimed is:

1. An article made of iron-'chromium-nickel alloy which is required tomaintain ductility and resistance to corrosion and which is subjected totemperature within the range of 900 degrees to 1350 degrees F. duringmanufacture or use whereby carbon is precipitated, said alloy being hotworked down to a finishing temperature between 1650 degrees and 1900degrees F. to produce a fine grain structure and then cooled, the alloybeing in ductile condition and having such fine grain structure thatwhen the carbon is precipitated in the form of carbide in the grainboundaries the carbide particles occupy only a minor portion of theextent of the boundaries, whereby the metal remains substantiallyresistant to intergranular corrosion.

2. An article of low carbon austenitic ironchromium-nickel' alloy whichis required to maintain ductility and which is subjected to temperaturewithin the range of 900 degrees to 1350 degrees F. during manufacture oruse whereby carbon is precipitated, said alloy being hot worked down toa finishing temperature between 1650 degrees and 1900 degrees F. toproduce a fine grain structure and then cooled, the alloy being inductile condition and of such small grain size that when the carbon isprecipitated in the form of carbide in the grain boundaries the carbideparticles occupy only a minor por-' tion of the extent of theboundaries, whereby the metal remains ductile.

3. An article of low carbon austenitic ironchromium-nickel alloy whichis required to main tain ductility and resistance to corrosion and whichis subjected to temperature within the range of 900 degrees to 1350degrees F. during manufacture oruse whereby carbon is precipitated, saidalloy being hot worked down to a finishing temperature between 1650degrees and 1900 degrees F. to produce a fine grain structure and thencooled, the alloy being in ductile condition and of such small grainsize-that when the carbon is precipitated in the form of carbide in thegrain boundariesthe proportion of carbide particles to the extent of thegrain boundaries is not suflicient to render the metal subject tointergranular corrosion.

low carbon austenitic

